At different stages of the athlete’s development there are different size windows of adaptation. The window of adaptation concept has some profound implications for directing training based on the athletes training age, level of development and biological age. For the younger developing athlete, especially the male at peak height velocity (growth spurt) the window of adaptation is huge. Conversely for the high level elite athlete who has an advanced training age the window of adaptation is quite small.
So what are the training implications of this? For the developing athlete it means that they can handle and in fact thrive on larger more general workloads. They have an anabolic advantage (The young female athlete less so) that allows them to recover quickly and handle the workload. There is greater margin of error. With the developing athlete, to some extent, anything you do will make them better. That is why you see some ridiculous programs achieve success with athletes at this stage of development. I do not condone that; I think you should be thinking long term and how the work done when they are developing will eradicate all deficiencies so they arrive at the elite level fully prepared to thrive.
At the elite level because of the accumulation of work the training should be very specific and directed. For the high level athlete the window of adaptation is quite small, so the loads must be more intense, very focused and specific to the individual. At the elite level more general work is not better, in fact it can be counterproductive. Because the window is quite small there is virtually no margin of error, therefore detailed training monitoring and planning are essential.